O Level Chemistry & IP Chemistry Notes by 10 Year Series Author - Chemistry Specialist

O Level Chemistry Made Easy

O Level Chemistry: Energy Changes (Exo/Endo) & Bond Energy

During the Intensive Revision Bootcamp, the concepts surrounding the chapter on Energy Changes (Endothermic/Exothermic) are dissected and the correct concepts are shared with the class.This chapter requires little memorization and in fact, 95% of the chapter involved the understanding of the concepts and applying them accordingly in application questions, especially for those that involves Bond-Forming & Bond-Making.

First we need to define Exothermic and Endothermic properly.

Exothermic Reactions:
– Reactions that gives out heat energy to the surroundings
– T of reaction mixture rises. Container feels hotter

Endothermic Reactions:
– Reactions that takes in heat energy from the surroundings
– Temperature of reaction mixture falls. Container feels cold.

Energy changes in reactions are caused by the making & breaking of chemical bonds

? Bond Making –> Exothermic Process

? Bond Breaking –> Endothermic Process

Tell if a Reaction is Exothermic or Endothermic by:

? Exothermic Reaction:? ?H bond breaking < ?H bond making

? Endothermic Reaction:? ?H bond breaking >? ?H bond making

Q) Is this reaction Exothermic or Endothermic then? What is the value of Enthalpy Change?
energy-changes-question.JPG
Given:
Cl-Cl bond energy = 242 kJ/mol
H-H bond energy = 436 kJ/mol
H-Cl bond energy = 431 kJ/mol

Any takers? Write down your suggested answers in the Comment section below, and we will discuss it accordingly.

PS: Once again –> Participation is Impt! No point me showing you all the answers and working, and u dont learn.

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Sean Chua
Many know me as the Ten Year Series book author for JC A-Level H2 Chemistry and O-Level Pure Chemistry. My 19+ years of coaching experience (since 1999) with more than 1500 students from 180+ JCs and Secondary Schools has allowed me to understand the true reasons why students are not able to perform well in Chemistry. Most importantly, my strength lies in using everyday analogies (even grandmothers can understand!) to simplify abstract concepts. Also, my teaching methodology has been designed to be fun, enjoyable and effective for my students. If you want to have an enjoyable time learning Chemistry and being motivated to excel in Chemistry, contact me today at 98287357

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Comments

  1. No. We don’t need to learn/memorise the bond energies of any chemical bonds.
    For students taking the GCE O-Level Pure Chemistry syllabus, the bond energies will be given in the question.
    For students taking the GCE A-Level H2 Chemistry syllabus, the bond energies could be found in the Data Booklet given in examination.

    Hope that helps 🙂

  3. How can we calculate the enthalpy of a reaction for the given equation when delta h value is not given??

  6. Pls I would like to know if an exothermic reaction can be reversed and then the same amount of heat regenerated over and over again? and if it is possible what comination of chemicals would one require for that? Looking forward to hearing from you. Thanks for your anticipated answer.

  7. i am pakistani u ? any website which tell about questions about electrolysis ,helping to prepare tests plz thanx

  8. reaction is exothermic bc its a bond making reaction and energy of bond formation 862kj is greater than bond breaking.

  9. from the given information

    H-H bond’s energy=436 kj
    Cl-Cl bond’s energy=242 kj

    o total energy on reactants side= 436+242

    =678 kj

    similarly,

    H-Cl bond’s energy=431 kj

    so total enery on products side =2 x 431

    =862 kj

    so by the formula del H rteactants-del H products

    678-862= -184 and “-” sign indicates an exothermic reaction as total energy of bond forming is greater than total energy of bond breaking………….. am i corect??????????

  10. Hi Brinda,

    Well done. Your answer is correct and i like the way you present it.

    Examiners marking your exam papers will be equally happy to see it too.

    PS: Just note that units ?kj/mol? need to be reflected at your final answer of ?-184 kj/mol?. If not marks will be taken away. It seems like you made the same error like Gary. Rmbr to reflect the units when showing your final answer/working.

    Hope this helps.

    Cheers
    Sean
    Master Trainer
    Author

  11. Hi Gary,

    Well done. Your answer is correct and i like the way you present it.

    Examiners marking your exam papers will be equally happy to see it too.

    PS: Just note that units “kj/mol” need to be reflected at your final answer of “-184 kj/mol”. If not marks will be taken away.

    Hope this helps.

    Cheers
    Sean
    Master Trainer
    Author

  12. Hi Amanda,

    Yes. You are absolutely spot on.

    It is indeed an Exothermic Rxns.

    Might be better if you show the proper working step by step in Chemistry Exams.

    Examiners are looking for the word “Overall Heat Change” in the chemical reaction is “-184 kj/mol”.

    Hope this is clear
    Sean
    Master Trainer
    Author

    PS: Keep up the good work.

  13. Energy required to break H-H and Cl-Cl bond= (+436) + (+242) = +678
    Energy required to form HCl bond=2 (-431) =-862

    Overall heat change= (+678) + (-862)
    =-184
    The reaction is exothermic because the amont of energy released during bond forming is greater than energy taken in during bond breaking.

  14. Total Energy required to break H-H and Cl-Cl bond = (+436) + (+242)
    = +678KJ

    Total energy released during bond forming = 2 x (-431)
    = -862

    Overall heat change = (+678) + (-862)
    = -184

    The reaction is exothermic because the amount of energy released during bond forming is greater than the energy taken in during bond breaking.

  15. Total energy used for breaking bond is 678 while energy for forming bonds is 862. So reaction is… exothermic?

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